This invention relates to zirconia-containing thermal barrier coatings having reduced thermal conductivity that comprise a stabilizer metal oxide selected from ytterbia, neodymia or mixture thereof, with or without lanthana, to stabilize the zirconia in the cubic crystalline phase. This invention particularly relates to such reduced thermal conductivity coatings having a zirconia-containing lower layer stabilized in the tetragonal phase for improved adherence to the bond coat. This invention further relates to articles having such coatings and methods for preparing such coatings for the article.
Components operating in the gas path environment of gas turbine engines are typically subjected to significant temperature extremes and degradation by oxidizing and corrosive environments. Environmental coatings and especially thermal barrier coatings are an important element in current and future gas turbine engine designs, as well as other articles that are expected to operate at or be exposed to high temperatures, and thus cause the thermal barrier coating to be subjected to high surface temperatures. Examples of turbine engine parts and components for which such thermal barrier coatings are desirable include turbine blades and vanes, turbine shrouds, buckets, nozzles, combustion liners and deflectors, and the like. These thermal barrier coatings typically comprise the external portion or surface of these components are usually deposited onto a metal substrate (or more typically onto a bond coat layer on the metal substrate for better adherence) from which the part or component is formed to reduce heat flow (i.e., provide thermal insulation) and to limit (reduce) the operating temperature the underlying metal substrate of these parts and components is subjected to. This metal substrate typically comprises a metal alloy such as a nickel, cobalt, and/or iron based alloy (e.g., a high temperature superalloy).
For reduced thermal conductivity, the thermal barrier coating is usually prepared from a ceramic material, such as zirconia. It is also desirable to chemically phase-stabilize the zirconia-containing thermal barrier coating in the cubic phase. The cubic phase is desirable for lower thermal conductivity and can be either fluorite or pyrochlore structure. Examples of such chemically phase-stabilized zirconias include yttria-stabilized zirconia, scandia-stabilized zirconia, ceria-stabilized zirconia, calcia-stabilized zirconia, and magnesia-stabilized zirconia. The thermal barrier coating of choice is typically a yttria-stabilized zirconia ceramic coating. A representative yttria-stabilized zirconia thermal barrier coating usually comprises about 7 weight % yttria and about 93 weight % zirconia.
There a variety of ways to further reduce the thermal conductivity of such thermal barrier coatings. One is to increase the thickness of the coating. However, thicker thermal barrier coatings suffer from weight and cost concerns. Another approach is to reduce the inherent thermal conductivity of the coating. One effective way to do this is to provide a layered structure such as is found in thermal sprayed coatings, e.g., air plasma spraying coatings. However, coatings formed by physical vapor deposition (PVD), such as electron beam physical vapor deposition (EB-PVD), that have a columnar structure are typically more suitable for turbine airfoil applications (e.g., blades and vanes) to provide strain tolerant, as well as erosion and impact resistant coatings.
Another general approach is to make compositional changes to the zirconia-containing ceramic composition used to form the thermal barrier coating. A variety of theories guide these approaches, such as: (1) alloying the zirconia lattice with other metal oxides to introduce phonon scattering defects, or at higher concentration levels, provide very complex crystal structures; (2) providing “coloring agents” that absorb radiated energy; and (3) controlling the porosity morphology of the coating. All of these approaches have limitations. For example, modifying. the zirconia lattice, and in particular achieving a complex crystal structure, limits the potential options for chemical modification and can interfere with good spallation resistance and particle erosion resistance of the thermal barrier coating.
Modification of the zirconia lattice to improve the reduction in thermal conductivity can also interfer with other mechanical properties of the thermal barrier coating. In particular, modification of the thermal barrier coating to reduce thermal conductivity can result in a coating that is less adherent to the underlying bond coat layer. As a result, the thermal barrier coating can be vulnerable to spallation over time, especially due to temperature cycling that occurs during normal operation of gas turbine engines. Inclusion of a lower layer comprising, for example, yttria-stabilized zirconia can improve adherence of the thermal barrier coating to the bond coat, but at the cost of losing some of the reduced thermal conductivity benefit achieved by changing the compositional parameters of the thermal barrier coating.
Accordingly, it would be desirable to be able to change the compositional parameters of the zirconia-containing ceramic composition to achieve further reductions in the thermal conductivity of the resultant thermal barrier coating by being able to stabilize the zirconia in the cubic crystalline phase. It would further be desirable to be able to achieve such reductions in thermal conductivity without sacrificing other desired mechanical properties of the thermal barrier coating, especially good spallation resistance.